In general, a thermal recording material is obtained by mixing a normally colorless or pale colored electron donating colorless dye and a phenolic compound, each dispersed to fine particles, adding a binder, a filler, a sensitizer, a slip agent, and other additives to form a coating color, and coating the coating color on a substrate such as paper, synthetic paper, films, or plastics, which develops a color by a momentary chemical reaction caused by heating with a thermal head, a hot stamp, or laser light to obtain a visible recording.
Thermal recording material is applied in a wide variety of areas such as measuring recorders, terminal printers for computers, facsimiles, and bar code labels. However, with recent diversification of recording devices for thermal recording materials and advance towards higher performance, quality requirements for thermal recording material have become higher and more difficult to achieve. For example, for high-speed recording, a thermal recording material which can provide a high recording density even with a small thermal energy is in demand. On the other hand, in view of storage stability of recording material, a thermal recording material is required which is superior in light resistance, oil resistance, water resistance, and solvent resistance.
Further, with the popularization of plain paper recording system such as electrophotographic or ink-jet systems, the thermal recording system has become often compared with these plain paper recording systems. For this reason, for example, stability of recorded portion (image) of the thermal recording material, or stability of unrecorded portion (background portion or white portion) before and after recording are required to be closer in quality to those of plain paper recording. Above all, the thermal recording material is required to have background color stabilities for heat, and for solvent (hereinafter referred respectively to as background color heat resistance, and background color solvent resistance). In other words, the recording material is required to have a "thermal sensitivity" only during recording, and a "thermal insensitivity" in other than recording.
As to the background color stability of thermal recording material, for example, Japanese Patent Laid-open Publication (OPI) 04-353490 discloses a thermal recording material having a relatively good background color stability even at a high temperature of about 90.degree. C. This thermal recording material contains 4-hydroxydiphenylsulfone compound and a metal salt of phosphate.
The background color stability of the thermal recording material disclosed in Japanese OPI 04-353490 is a Macbeth density of background color of about 0.11 when treated in a dryer at 95.degree. C. for 5 hours, which is fairly good, however, it has been still insufficient for stability at higher temperatures, for example, 150.degree. C.
Therefore, an object of the present invention is to provide a thermal recording material which is superior in background color stability.